This invention relates to photoconductive image pick-up tube targets and, more particularly, to a heat-resistant structure of the photoconductive layer. More specifically, the invention concerns a photoconductive image pick-up tube target which can prevent deterioration of the sensitivity characteristic in use at high temperatures.
Recently, a new photoconductive image pick-up tube target of a rectifying contact type, in which a rectifying contact is formed between a P-type photoconductive film containing amorphous selenium (Se), arsenic (As), tellurium (Te), etc. and an N-type semiconductor film, has been proposed as disclosed in Japanese Patent Preliminary Publication No. 24619/1974 and Japanese Patent Application No. 45198/1975. This type of image pick-up tube target has such advantages as minimal residual image and flare, high resolution, minimal white dot image defect and simple manufacture and has a construction as will be described below. As shown in FIG. 1, it includes a disc-like glass plate 1 made of transparent glass, a transparent electrode 2a formed on the base plate 1 and mainly composed of an oxide of an element selected from a group consisting of tin, indium and titanium, and a very thin N-type semiconductor film 2b formed on the transparent electrode 2a and made of a material selected from ZnSe, GeO and CeO.sub.2. The transparent electrode 2a and N-type semiconductor film 2b form a transparent semiconductor film 2. A P-type photoconductive film 3 includes a first layer 3a of a thickness of about 300 .ANG. formed on the transparent semiconductor film 2 in rectifying contact therewith and containing 95% by weight of Se and 5% by weight of As as shown in region A in FIG. 2, and a second layer 3b of a thickness of about 600 .ANG. formed on the first layer 3a and containing 65% by weight of Se, 5% by weight of As and 30% by weight of Te as shown in region B in FIG. 2. The second layer 3b serves as a sensitizer layer in which Te concentration sharply rises at a film thickness of about 300 .ANG., i.e., the termination of the first layer 3a and sharply falls down at a film thickness of about 900 .ANG.. Concentration of As is the same over a thickness of about 900 .ANG. covering the first and second layers 3a and 3b to increase thermal stability of Se. The P-type photoconductive film 3 further includes a third layer 3c formed on the second layer 3b to a thickness of about 600 .ANG. and containing Se varying in concentration from 70% by weight to 100% by weight and As varying in concentration from 30% by weight to 0% by weight as shown in region C in FIG. 2. Since the third layer 3c contains As whose concentration gradient varies linearly from lower to higher over a thickness of about 600 .ANG., it can enhance the sensitizing effect of the second layer 3b and further increase the thermal stability of Se. Further, a semiconductor film 4 of Sb.sub.2 S.sub.3 is vapor deposited on the P-type photoconductive film 3 to a thickness of about 1,000 .ANG., and serves to assist the landing of scanning beams. Incident light 5 is irradiated from the side of the glass plate 1, while a scanning electron beam 6 is applied to the semiconductor film 4.
With the image pick-up target having the above construction, the objective of increasing the sensitivity with respect to long wavelength light can be attained, and properties generally required for the image pick-up tube such as residual image and high-light sticking can be satisfied.
However, while the image pick-up target of the construction provides satsifactory performance under normal working conditions, the sensitivity characteristic with respect to long wavelength light is extremely degraded when the ambient temperature is greatly increased from the ordinary working temperature.